The return of director Bryan Singer to the “X-Men” franchise for the next film won’t be the only connection to the original “X’ movies re-established in “Days of Future Past.”

Patrick Stewart and Ian McKellan, who played antagonists Xavier and Magneto in the original “X-Men” trilogy, will return in the sequel to last year’s “X-Men: First Class,” Singer wrote on his Twitter account this week. Singer directed 2000’s “X-Men” and 2003’s “X2: X-Men United,” and has signed on to return for “Days of Future Past.”

Both “First Class” and the new movie portray Xavier, Magneto and the other characters as younger, with James McAvoy and Michael Fassbender playing Stewart and McKellan’s characters, so it’s unclear how exactly the film will handle two of each character from a storytelling standpoint. Time travel, perhaps, along the lines of the loophole that allowed Zachary Quinto and Leonard Nimoy to both play Spock in the same “Star Trek” movie?